1. Field of the Invention
This invention relates generally to perpendicular magnetic recording media, and more particularly to a disk with a perpendicular magnetic recording layer for use in magnetic recording hard disk drives.
2. Description of the Related Art
Perpendicular magnetic recording, wherein the recorded bits are stored in a perpendicular or out-of-plane orientation in the recording layer, is a promising path toward ultra-high recording densities in magnetic recording hard disk drives. The most common type of perpendicular magnetic recording system is one that uses a “probe” or single pole recording head with a “dual-layer” media as the recording disk, as shown in FIG. 1. The dual-layer media comprises a perpendicular magnetic data recording layer (RL) formed on a “soft” or relatively low-coercivity magnetically permeable underlayer (SUL), with the SUL serving as a flux return path for the field from the pole recording head. This type of system is also called “Type 1” perpendicular magnetic recording. In FIG. 1, the RL is illustrated with perpendicularly recorded or magnetized regions, with adjacent regions having opposite magnetization directions, as represented by the arrows. The magnetic transitions between adjacent oppositely-directed magnetized regions are detectable by the read element or head as the recorded bits.
FIG. 2 is a schematic of a cross-section of a prior art perpendicular magnetic recording disk showing the write field H acting on the recording layer RL. The disk also includes the hard disk substrate, a seed or onset layer (OL) for growth of the SUL, an exchange break layer (EBL) to break the magnetic exchange coupling between the magnetically permeable films of the SUL and the RL and to facilitate epitaxial growth of the RL, and a protective overcoat (OC). As shown in FIG. 2, the RL is located inside the gap of the “apparent” recording head (ARH), which allows for significantly higher write fields compared to longitudinal or in-plane recording. The ARH comprises the write pole (FIG. 1) which is the real write head (RWH) above the disk, and an effective secondary write pole (SWP) beneath the RL. The SWP is facilitated by the SUL, which is decoupled from the RL by the EBL and by virtue of its high permeability produces a magnetic mirror image of the RWH during the write process. This effectively brings the RL into the gap of the ARH and allows for a large write field H inside the RL.
One type of material for the RL is a conventional polycrystalline granular cobalt alloy, such as a CoPtCr alloy. This conventional material has out-of-plane of perpendicular magnetic anisotropy as a result of the c-axis of its hexagonal-close-packed (hcp) crystalline structure being induced to grow perpendicular to the plane of the layer during deposition. To induce this growth, the EBL onto which the RL is formed is also typically a material with an hcp crystalline structure. Thus ruthenium (Ru) is one type of material proposed for the EBL.
A perpendicular magnetic recording medium has been proposed wherein the RL is an antiferromagnetically-coupled (AFC) layer of two identical ferromagnetic layers, each having perpendicular magnetic anisotropy, separated by an antiferromagnetically (AF) coupling layer. In this type of medium, as described in U.S. Pat. No. 6,815,082 B2, the first or lower ferromagnetic layer and the second or upper ferromagnetic layer have the same composition and are formed of a conventional polycrystalline granular cobalt alloy. Thus in a perpendicular magnetic recording medium with an AFC RL, the EBL would also have to have an hcp crystalline structure to induce the perpendicular magnetic anisotropy of the lower layer in the AFC layer. This type of medium is depicted schematically in FIG. 3.
The best performance for writing perpendicular magnetic recording is obtained when the EBL is as thin as possible, i.e., the minimum thickness required to provide magnetic decoupling of the SUL and the RL, so that flux can readily pass through the EBL during the write process. However, while a reduction in thickness of the EBL is desirable, the EBL must have a thickness sufficient to provide the template for the growth of the cobalt alloy RL to assure its c-axis is perpendicular. A relatively thick Ru EBL is also required to provide an RL with high coercivity and low enough inter-granular exchange coupling to minimize the intrinsic media noise. Thus, if Ru is used as the EBL it must be at least approximately 80 Angstroms thick for current RL materials.
What is needed is a perpendicular magnetic recording medium with an AFC recording and a substantially thinner effective EBL.